1. Field of the Invention
The present invention relates to sheet films having pressure control valve portions therein, a packaging material comprising said sheet film, and packages including packages for various drinks and foods, comprising the sheet film and the packaging material. The package of the present invention is a package wherein foods and beverages can be stored and a simply cooked in a cooking device such as a microwave oven. More specifically, the sheet or film of the present invention relates to a sheet film having two or three layers comprising a first film layer and second, and if necessary third film layers having one or more holes. The sheet film having a pressure valve portions can also comprise a single layer having one or more holes passing partially through the sheet film on one side, and a thin portion covering each of the one or more holes. Also in accordance with the invention, a packaging material is formed with the sheet film, and packages, including sealed packages for foods and beverages are made using the packaging material. The package material of the invention is capable of releasing pressure when a predetermined pressure is reached inside the package, or a maximum sustainable pressure differential across the sheet film is exceeded.
2. Description of the Background Art
Current eating habits are undergoing great changes. Particularly, the use of precooked foods and beverages and semi-cooked foods and beverages is increasing. One example is frozen foods, also called retort foods, wherein cooked food is put in a container and then kept frozen. Frozen foods typically only require heating to be ready for eating. The number of foods and beverages which can be cooked in a short period of time and with ease by using microwave ovens and other cooking apparatuses is increasing.
Sheet films for packaging foods and beverages include sheet materials made of synthetic plastic films, paper, metallic foils, or a sheet material having a multi-laminated structure of such materials. The material properties and functions of such packages differ depending on their objectives, such as, preservation, storage, transportation, distribution, display, and depending on the method and type of maintaining freshness, the environment of the foods and beverages, as well as whether the foods and beverages are fresh or processed. Therefore, it is necessary to select and use appropriate packaging materials having material properties and configuration in conformity with such objectives.
When cooking apparatuses, such as microwave ovens or electro-magnetic cookers are used to rapidly heat packaged foods and beverages, the problem of having a rapid increase in pressure is encountered. In this case, as a pressure safety measure, a method is commonly employed wherein a hole is provided in part of a package, the hole is sealed by a seal having an adhesive, and then the seal is pealed off before heating in a microwave oven and the like. In another commonly employed method, a hole is provided in a package and a seal having a pressure regulation valve function is provided in the hole. In a further method, portions having a high melting point and low melting point are provided in the seal portion of a package whereby when the portion having a low melting point melts to release the pressure. Lastly, there is a method wherein a hole is made in a package or a portion of a package is cut before the package is heated in a microwave oven or the like.
For example, package containers to be used in making popcorn in a microwave oven are disclosed in Utility Patent Publications Sho. 63-49187 and Hei. 1-25593. These containers are obtained by putting corn, edible oil and spices in a holding plate made of paper followed by sealing this holding plate with packaging materials made of synthetic plastic films. When the corn which is packed and sealed by a film is subjected to irradiation by a microwave oven, the corn is heated to become popcorn. In this case, a small hole for ventilation can be provided at the joined portion of the film, whereby explosion of the package can be avoided by releasing the pressure accumulated inside of the package through the hole.
When a sealed package container is heated, the pressure inside generally increases. A package for foods and beverages is known which makes it possible to cook foods and beverages in a short period of time by using a microwave oven. Namely, in Japanese Patent Publication Hei. 7-22547 and Japanese Laid-Open Patent Hei. 6-329179, a package for foods and beverages is disclosed wherein sheet type pressure regulation valves are joined to a package for foods and beverages whereby such pressure regulation valves are opened to make it possible to regulate the pressure inside of the package when the pressure inside of the package becomes higher than a specified pressure.
However, although the pressure increase inside of the package can be actually avoided by such prior art packages for foods and beverages, in reality, it is not possible to regulate the pressure in a precise way. In the case of popcorn, popcorn of the same quality can not be easily obtained due to the difference of the pressure increase in each package during irradiation in the microwave oven, because uniformity in the size of holes provided in each package can not be easily obtained, and, in the extreme case, the diameter of the holes differs from package to package. Furthermore, when a sheet type pressure regulation valve is employed, the cost may increase due to the increased production processing for the manufacture of pressure regulation valves, and the workmanship and taste of the foods and beverages after being cooked may differ from package to package. This difference may be due to variations, such as the minute variation of the attached position of the sheet type regulation safety valve, or variation of the adhesive strength between the regulation safety valve and the sheet.
The present invention provides, at a low cost, a sheet for packaging suitable for various different objectives, a packaging material using the same, and packages, including sealed packages for foods and beverages. Particularly, the present invention provides a sheet packaging material and packages for foods and beverages using the sheet, which eliminate the above-mentioned problems of prior art packages for foods and beverages. More particularly, the present invention provides a sheet material of reliable quality, which is easy and inexpensive to manufacture into packaging materials.
The present invention relates to packages for foods and beverages to be used in various processing treatments, such as retort, cook-chilled, cook-serve, cooking in vacuum, and freezing, wherein the package has a pressure regulation valve function as well as the conventional packaging function. One example of the application of the present invention is a package wherein a heating environment suitable for each package of food and beverage can be provided when rapid heating is employed by using a microwave oven or an electromagnetic cooker. In such a package, foods and beverages can be heated without causing any unevenness in the temperature and at the same time the pressure due to the heating can be automatically regulated. Another example of the present invention is a package for foods and beverages wherein fresh foods and beverages, and spices, if desired, can be cooked in an appropriate environment, such as the heating temperature, humidity, pressure and time, inside of the package.